Microclimates are very important to anyone with an interest in growing cold hardy tropical plants. A Microclimate is an area where the climate differs from the surrounding area. Microclimates can be very small such as the south side of a house or larger such as a densely built up city that has a different climate than the nearby countryside. Some Microclimate differences can be dramatic. New York City is a good example of an area with a very pronounced microclimate. New York City on the latest National Arbor Day Foundation Climate Zone Map is shown as Zone 8. The average minimum winter temperature in a zone 8 climate is 20-10 degrees Fahrenheit. This is the same climate zone as Atlanta Georgia.
Many species of subtropical plants can grow in a Zone 8 Climate. These include species of palms, many yuccas, Southern Magnolia’s, Crape Myrtle etc.
In New York City’s case there are several reasons for its warmer climate compared to other cities at a similar latitude such as Columbus Ohio or Indianapolis Indiana. The first is that New York City is at sea level. It will be several degrees warmer on average than a slightly higher city at the same latitude. The second reason is that it is surrounded by water. Water warms slowly and cools slowly compared to land. This means their summers will be slightly cooler and their winters slightly warmer than cities that are inland. The final reason for their warmer microclimates is the urban heat island effect. This happens when built up areas absorb and hold heat better than open countryside or forests. Other locations on the east coast can have warmer microclimates for similar reasons. Philadelphia, Cape Cod, Nantucket Island in Massachusetts and Block Island in Rhode Island all come to mind.
You can use Microclimates to your advantage in growing cold hardy tropicals. The first example would be a south facing wall of a house. Another example would be a house on the side of a hill. The person living on the slope has less chance of late frost in the spring or early frost in the fall as opposed to the person down in the valley. This is because cold air is heavier and flows downhill. Other simple features can influence microclimates. An area right by a dryer vent can have soil that never freezes very deep even in the coldest of winters allowing plants such as Canna Lilies to survive much farther north than normal as the roots never freeze out. Growing up we had a large patch of Canna Liles surrounding our dryer vent. They were never dug up and survived the winter just fine. This was in an area of zone 6 winters. Normally Canna’s require zone 7-8 climates to survive the winter unprotected.
Microclimates can be created. Darker surfaces as generally warmer than lighter surfaces. Having black plastic mulch can raise soil temperatures by several degrees and in some cases protect lower growing plants from moderate frosts. Of Course the ultimate in creating your own Microclimates are various forms of winter protection such as cold frames and Greenhouses. I will touch on more of these types of winter protection in a future post.